The present invention relates to a switching power supply of two-transistor-forward type or two-transistor-flyback type which is cyclically connected and disconnected to an inductive load, e.g., a transformer, by switching means such as transistors, and more specifically to a snubber circuit of the switching power supply for absorbing counter-electromotive force generated from the inductive load when the switching means are turned off.
One method of obtaining rectified direct current output with desired voltage from a direct current power supply is to provide a switching power supply of two-transistor-forward type or two-transistor-flyback type with a transformer. The transformer is cyclically connected and disconnected to the direct current power supply by two switching means at a duty rate that sets output voltage of a primary coil of the transformer at a constant value. The switching power supply is provided with a snubber circuit for absorbing counter-electromotive force. Because recently developed transistors, which serve as the switching means, show very short turn-off time of 0.1.mu. sec, a very steep counter-electromotive force is generated from the transformer as an inductive load when the transformer is disconnected by turning off the transistors. The transient voltage spike can potentially damage the transistors. An example of the switching power supply that incorporates a prior art snubber circuit is shown in FIG. 7.
FIG. 7 shows a circuit diagram of a two-transistor-forward type switching power supply that is equipped with two transistors 2 and 3 acting as switching means, which transistors are represented by single-contact switches in the figure. In FIG. 7, the switching power supply generates a predetermined direct current output voltage Vo from a voltage Vi of a direct current power supply 1. Power is fed from the power supply 1 to a primary coil 7a of a transformer 7 for voltage transformation via transistors 2 and 3 which are turned on and off simultaneously at a duty rate corresponding to the output voltage Vo. An output circuit, connected to a secondary coil 7b of the transformer 7 which is in phase with the primary coil 7a, comprises a diode 8 for rectifying alternating voltage form the secondary coil 7b, a free-wheeling diode 9, a smoothing reactor 10, and a large capacity smoothing capacitor 11.
On the side of the primary coil 7a, diodes 4 and 5 which absorb excess counter-electromotive force generated when the transistors 2 and 3 are turning off, are connected between both ends of the primary coil 7a and both ends of the power supply 1 in a connection as shown in FIG. 7. When the counter-electromotive force exceeding the power supply voltage Vi is generated in the primary coil 7a, the diodes 4 and 5 become conductive to absorb the excess counter-electromotive force by regenerating the excess portion of the counter-electromotive force. However, the diodes 4 and 5 cannot immediately absorb the quickly rising counter-electromotive force from the primary coil 7a, which counter-electromotive force starts increasing around 0.1.mu. sec after the transistors 2 and 3 are turned off. The diodes 4 and 5 require a so-called forward recovery period of around 0.5.mu. sec, during which the diodes 4 and 5 are not conductive even if the voltage applied to the diodes 4 and 5 becomes inverted, i.e., the voltage applied to the diodes changes direction from reverse to forward.
To absorb the transient voltage spike, a snubber circuit 6, which incorporates a capacitor 61 and a resistor 62, is connected between the transistors 2 and 3 in parallel to the primary coil 7a. By employing the capacitor 61 of a large capacity and the resistor 62 of a small resistance, the counter-electromotive force present in the snubber circuit 6 immediately after the transistors 2 and 3 are turned off, are absorbed. Accordingly, the transistors 2 and 3 are protected from damage or break-down, and noise caused by the steep counter-electromotive force and fed to outside circuits may be suppressed, even when the slope or the peak value of the counter-electromotive force generated from the primary coil 7 is large.
The snubber circuit of FIG. 7 suffers from a characteristic turn-on loss of the transistors because a current flows through the snubber circuit not only when the transistors are turned off, but also when the transistors are turned on. In FIG. 7, when the transistors 2 and 3 are simultaneously turned on, a closed circuit is established between the terminals of the power supply 1 via the transistor 2, the capacitor 61, the resistor 62, and the transistor 3. Through this closed circuit, a large transient current flows, which current is limited only by the resistor 62, thereby causing turn-on loss of the transistors 2 and 3. This transient current is a charge-up current of the capacitor 61 of the snubber circuit 6, which current may be reduced by employing the capacitor 61 of small capacity and the resistor 62 of large resistance. However, application of this transient-current reduction scheme is quite limited because this transient-current reduction scheme substantially negates the snubber circuit's ability to absorb the counter-electromotive force.
When the transistors 2 and 3 are turned off, the counter-electromotive force generated in the primary coil 7a is absorbed by the snubber circuit 6. The energy of leakage inductance and wiring inductance of the primary coil 7a is stored by charging the capacitor 61, and the energy stored in the capacitor 61 is consumed in the resistor 62 during the turning-off period of the transistors 2 and 3. Thus, the resistor 62 is provided for consuming the energy stored in the capacitor 61 in response to charging and discharging of the capacitor 61. The energy that should be consumed at every charging/discharging cycle is proportional to the capacitance of the capacitor 61 and to the square of the charging voltage.
To improve the ability of the snubber circuit, capacitance of the capacitor should be increased, in accordance with which the-amount of energy that should be consumed in the resistor 62 increases. Since the aforementioned turn-on loss of the transistors 2 and 3, and the power loss in the resistor 62, increase in proportion to on-off switching frequency of the transistors 2 and 3, switching frequency increase of the transistors 2 and 3 causes increase in temperature of the resistor 62. Accordingly, the prior art snubber circuit is not suitable for high frequency switching.
It is an object of the present invention to provide a snubber circuit for a switching power supply which eliminates the turn-off and turn-on loss in the switching means and facilitates increased switching frequency.